As far as I understand by reading the data sheet, the RS-232 input lines are 5 volt tolerant. Apparently the chip has a pin (4) called vdd_232 which you can use to set the voltage of output-lines, which would protect connected MCU's tolerating 3-3.3V max. So, yes, it should be possible.

Quote

"RS-232 VDD. The RS-232 output signals (Pin 1 ~ Pin3) are designed for 5V, 3.3V or 3Voperation. VDD_232 should be connected to the same power level of the RS-232 interface. (The RS-232input signals are always 5V~3V tolerant.) Note: This document version only provides 5V DC characteristic information. Refer to future revisions for updates. (Document Revision: 1.6Document Release: April 26, 2005) "

You're right, I shouldn't trust googles results that fast, after looking it up once it indeed was the wrong datasheet, sorry.

It's nice those input pins indeed are 5 volt tolerant, but.... I wonder how 5v-tolerant the vdd325-pin is.The HX-less version can take 5volt, but the selectable vdd325 voltage of the HX is 1.8-3.3v, applying 5 volts may be too much. An input pin on arduino will see anything below 0.5 volts as low and above ~2 volts as high though, it doesn't need 5v. I'd apply 3.3 volt to the vdd325 pin to stay within limits, with a good chance that it will work.